Atomic clocks have been developed since more than 50 years, following fundamental scientific progress and developments in the field of quantum mechanics and microwave spectroscopy. Electronic technology and control systems have made huge progress in the field of atomic clocks. They aim mainly to improve the degree of accuracy and also the stability of the frequency signals delivered by the atomic clocks. In recent years different types of atomic clocks have been developed such as the cold atomic fountain clocks. Also, new configurations have been developed to make atomic clocks airborne (airplanes, satellites). In order to achieve miniaturization of atomic clocks the trend has been to use techniques such as double resonance (DR) microwaves or coherent population trapping (CPT). A Rubidium DR miniaturized clock has been described in: V. Venatraman et ak. “Micro fabricated Chip-Scale Rubidium Plasma Light Source for Miniature Atomic Clocks”, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 59, 3, pp. 448-456, 2012. The described chip-scale miniature clock is based on a stack configuration, having a thickness of typically 1 cm, wherein all the components (light source, gas cell, detector) are piled on top of each other, limiting therefore the possibility to reduce considerably the overall thickness and size of the device.
CPT is a challenging technique to miniaturized atomic clocks. CPT is a nonlinear phenomenon in atoms in which coherences (electromagnetic dipole moments) between atomic energy levels are excited by pairs of optical fields. Atomic clocks based on CPT rely mainly on a vertical cavity surface emitting laser diode (VCSEL) as the light source.
CPT techniques, also called electromagnetically induced transparency techniques, are described in for example: S. Harris “Electromagnetically induced transparency”, Physics Today, p. 36-42, 1997, and also discussed in Knappe et al. “Characterization of coherent population-trapping resonances as atomic frequency references”; J. Opt. Soc. Am. B, pp. 1545-1553, 2001), both articles being incorporated herein by reference in their entirety.
In most vapor-cell frequency references, which do not use CPT, the minimum size of the clock physics package is determined in part by the cavity that confines the microwaves used to excite the atoms. Because the cavity has to be a resonant cavity, it is usually larger than half of the wavelength of the microwave radiation used to excite the atomic resonance. For cesium and rubidium vapors for example, this wavelength is typically several centimeters. This size is a fundamental limitation to develop vapor-cell references that would be suited for portable applications. The large gas volume needed for standard vapor-cell frequency references implies also a considerable challenge to maintain the package and the cell at the required temperature. Frequency standards should be useful over a wide range of temperatures. Also, the atomic transition frequency is dependent on the temperature of the vapor-cell, therefore the cell temperature must be controlled to a fixed value and have a great stability. A significant amount of power may be required to maintain the cell at a fixed temperature. This power depends on the cell size and requires several watts for cells having a volume of a cubic centimeter. Therefore there is a huge interest in developing cells with smaller sizes.
The coherent population trapping (CPT) technique does not require a microwave field applied to the gas in the cell and the performance of the atomic clock scales proportional to the size of the vapor cell. Atomic clocks based on CPT techniques may therefore be miniaturized, which also has the benefit of a simpler implementation, and so also leads to cheaper solutions as the CPT gas cells may be realized for example by batch processing. Recent advances in the research and development of CPT clocks have shown that very good stabilities may be obtained. For all the above mentioned aspects, CPT based clocks are a good choice to make miniaturized atomic clocks.
Prior art discloses a number of realizations that aim to reduce the size and especially the thickness of miniaturized DR, CPT or other types of atomic clocks. The document WO 2013/120334A1 for example discloses a miniaturized atomic clock comprising discrete optical elements such as beam splitters, lenses and prisms in order to reduce the overall size of the optical system. The system disclosed in WO 2013/120334A1 requires the alignment and assembly of the different optical components. The cost of such an atomic clock remains quite expensive because of the use of discrete optical components and their assembly. Also, the optical stability of the system is limited and shocks and vibrations may reduce the reliability of the device. The possible reduction in size of the device disclosed in WO 20131120334A1, mainly the reduction in thickness of the system, is basically limited by the use of discrete optical components.
In another approach, the application US 201410014826A1, discloses a vacuum cell for an atomic clock comprising folded optics. In US 201410014826A1 a set of diffractive optics is configured to reflect the optical beam within the enclosed volume of the gas cell of the atomic clock. The system disclosed in US 201410014826A1 requires to be coupled to a light beam having a light beam adapted to the incoupling diffractive optics. It would therefore be difficult to reduce the overall size of the system. Also, discrete components such as lenses or a fiber holder need to be adapted to the cell making the alignment and assembly of a complete miniature clock complicated and expensive.